The government has rights in this invention pursuant to Contract No. N66001-90-C-0162 awarded by DARPA.
This invention relates to a rotation sensor used for an advanced global positioning and inertial guidance system.
Optical rotation sensing devices include ring laser gyros, fiber optic rotation sensors, and the like. The fiber optic rotation sensor ordinarily comprises an interferometer which includes a light source, a beam splitter, a detector, and a light path which is mounted on a rotatable platform. Light from the light source is split by the beam splitter into two beams which are directed to opposite ends of the optical path and which then counterpropagate around that path. As the light beams exit the light path, they are recombined and the resulting combined light beam is sensed by a detector. A sensing circuit connected to the detector determines any phase difference between the counterpropagating light beams.
Assuming that this fiber optic rotation sensor experiences no rotation, ideally no difference in phase between the counterpropagating light beams will be detected. On the other hand, if the sensor experiences rotation, there will be a phase difference between the counterpropagating light beams which can be detected to indicate the extent and direction of rotation.
In a fiber optic rotation sensor, an optical fiber is coiled, usually in multiple layers, around a spool, with each layer containing multiple turns. Currently, such coils are typically wound as quadrupoles. In order to form a quadrupole coil, each half of a continuous optical fiber is first wound onto respective intermediate spools. The first spool is then used to wind a first layer of turns in a clockwise direction around a sensor spool. This first layer is wound around the sensor spool from the first end to the second end of the sensor spool. The second spool is then used to wind a second layer of turns in a counterclockwise direction around a sensor spool. This second layer is wound around the sensor spool from the first end to the second end of the sensor spool. The fiber on the second spool is then wound back from the second end to the first end of the sensor spool to form a third layer. The first spool is then used to wind a fourth layer of turns from the second end of the spool to the first end. Thus, a portion of one half (i.e. one end) of the optical fiber is used to form the first and fourth layers of turns and a portion of the other half (i.e. the other end) is used to form the second and third layers. These four layers of turns are usually referred to as a quadrupole. If "+" and "-" are used to designate the first and second halves or ends of the optical fiber respectively, this quadrupole is wound with +--+ layers. The quadrupole is repeated for as many layers as is desired for the optical path. Accordingly, a second quadrupole will be wound with +--+ layers about the first quadrupole so that the resulting two quadrupole arrangement will have a +--+ +--+ layer configuration.
When a fiber optic coil wound in this fashion is subjected to an axial and/or radial time varying temperature gradient, there will be a phase difference between the counterpropagating light beams which results in a false indication of rotation; that is, this phase difference is an error. Causes other than axial and/or radial time varying temperature gradients can produce errors which may result in a false indication of rotation. For example, errors can result if the layers of the coil are wound inconsistently in the axial and/or radial directions and if the layers are subjected to varying environmental conditions such as a time varying temperature gradient. Thus, although the present invention is discussed in terms of errors produced by axial and/or radial time varying temperature gradients, the present invention is useful in substantially reducing errors resulting from other axial and/or radial influences as well. Consequently, errors resulting from axial and/or radial influences are referred to herein as axial and/or radial errors.